Infection with persistent viruses such as herpesviruses, hepatitis B and C and HIV presents a significant clinical problem. T cell exhaustion can contribute to viral persistence or reactivation and it is therefore important to understand why this occurs and to identify the underlying defects in the immune response. Failure to control some of these viruses, including the gammaherpesviruses, is particularly problematic in immunocompromised individuals such as AIDS patients or transplant recipients who lack effective CD4 T cell function. Murine gammaherpesvirus-68 (MHV-68) is a naturally-occurring rodent pathogen, which is closely related to the human pathogens Epstein Barr virus (EBV) and Kaposi?s sarcoma-associated herpesvirus (KSHV). Infection of mice with MHV-68 provides a tractable and well-characterized small animal model for studying the immunological control of gammaherpesvirus infection and the effects of defective CD4 T cell help on other components of the immune system. CD4 T cells are not required for acute control of replicating MHV-68, but are required for effective long-term control. Our early work highlighted the importance of CD40-CD40L interactions in CD4 T cell help in the long-term control of MHV-68 and showed that CD40 stimulation alone, using agonistic antibodies, was sufficient to prevent viral reactivation in the lungs of CD4 T cell deficient mice. CD40 stimulation upregulates CD80 and 86 on dendritic cells, which interact with positive and negative costimulators CD28 and CTLA-4, respectively. Interestingly, we have shown that, although CD80 and 86 are essential for long-term control of MHV-68, CD28, the only known positive costimulatory receptor for CD80/86 is not required, suggesting the possibility of a novel receptor. CD28, CD80 and 86 are part of the CD28-B7 superfamily, which comprises several members that mediate positive or negative costimulation of T cells. In this application, we will investigate the mechanisms by which CD40 and CD28-B7 family members interact to regulate CD8 T cell responses during the response to gammaherpesvirus infection. In Aim 1 we will investigate why CD4 T cells are required for the long-term control of MHV-68, but are not essential for shortterm (acute) control of the virus. Our data suggests that two alternative pathways of CD8 T cell activation function during acute, but not during long-term control. In Aim 2 we will investigate the possibility that a novel receptor for CD80/86 (B7-1, 2) functions in the long-term control of MHV-68. In Aim 3, we will investigate the role of PD-1, an inhibitory B7 family member that we have shown is upregulated on CD8 T cells in the absence of CD4 T cells, in acute and long-term control of MHV-68. These studies will provide important insight into differences in the mechanisms by which CD8 T cells mediate acute and long-term control of persistent viral infections and may be of significant value in designing immunotherapeutic agents or vaccines to combat persistent infections. The results obtained may have broader significance in understanding the mechanisms controlling T cell activation or tolerance, in general.